1. Field of the Invention
The invention relates to a device for transmitting optical signals between units that are rotatable relative to each other. Devices of this kind are used preferably in computer tomographs.
2. Description of the Prior Art
Various devices are known for transmitting optical signals between units that are rotatable relative to each other, particularly those having an unobstructed inner diameter. A basic problem existing here is that of designing a means for carrying light along the circumference of the device, and also suitable means for coupling light in and out. For use in computer tomographs, devices of this kind must have large unobstructed diameters of an order of magnitude of 1 meter. The circumferential speed of a rotation may be of an order of magnitude of 20 m/s. At the same time, data rates of more than 1 gigabit per second (Gbaud) must be feasible.
Thus, the U.S. Pat. No. 4,109,997 discloses an optical rotating data transmission device in which travel of light along the circumference occurs by reflection at two opposite faces. Light guides or glass fibers are provided for coupling light in or out, a bundling or focusing of the light beam being effected by means of lenses. Wideband data transmission with period lengths of a modulation signal which are substantially smaller than the transit time of the light around the circumference of the device is not possible, because a multiple-path reception of signals occurs at positions of a receiver close to a transmitter. Thus, signals received from the transmitter along a short path, and signals which have been reflected at least once around the circumference of the device, are received simultaneously. The transit time difference must be small in comparison with the period length of the modulation signal. Thus, with an inner diameter of about 1 meter, a total transit time around the circumference of about 10 nanoseconds results. For example, in a transmission of digital signals this makes it possible to achieve bit periods of maximally 50 nanoseconds, corresponding to a maximum transmission rate of 20 Mbaud.
An improvement of the optical system is disclosed in U.S. Pat. No. 4,525,025. This illustrates, particularly in FIG. 10, a specially suitable trench for transmitting optical signals with low attenuation. It consists only of one part, and can therefore be manufactured at favorable cost. However, this patent specification describes no effective solution of the problem of bandwidth limitation. In addition, the proposed coupling-in or coupling-out of light by blunt fiber ends can be achieved only with an extremely poor efficiency. Thus, this device is suitable only for small diameters. This device is extremely compact, but requires fiber couplers that maintain polarization when dividing the light from one single transmitter among a plurality of fibers for the purpose of feeding-in.
An improvement of optical coupling-in or coupling-out is disclosed in U.S. Pat. No. 4,555,631. In this, the coupling-in of optical signals into a mirror-finished cylinder is effected by means of two mirrors. For coupling-out, an additional coupling-out element is provided to be disposed at a fixed position in the trench. However, here too a large attenuation of the optical transmission path results, because the coupling-in mirrors cannot be placed arbitrarily close to the mirror-finished cylinder, in particular at high speeds of movement. Finally, the problem of bandwidth limitation is not solved. Thus, the light is conducted from a coupling-in position to a coupling-out position along two paths in opposite directions, and finally jointly evaluated in a receiver. Here too the limitation applies that the period length of the modulation signal must be substantially smaller than the transit time of the light around the circumference of the device. Finally, the contour of the mirror-finished trench must be adapted to the mirrors.
A device having an especially high optical efficiency is described in U.S. Pat. No. 4,934,783. In this, a focusing of the beam of rays is effected through a lens system. However, this system involves much outlay, is expensive to fabricate, and is suitable only for small diameters. Furthermore, here too the wideband problem has not been solved.
DE 195 43 386 C1 describes a device for wideband signal transmission with a possibility of a large bandwidth, but gives no indication of transmission with high transmission quality.